1. Field of the Invention
The present invention relates to an optical integrated circuit device having a Y-branch structure such as a 1xc3x97N splitter (including a Y-branch waveguide circuit) or an optical transmission/reception module circuit.
2. Description of the Related Art
Publication: Japanese Unexamined Patent Publication No. 1994-18730.
The Y-branch structure is one of the essential technologies that are vital in achieving branching of a light signal. To date, a number of publications have disclosed numerous embodiments for realizing a Ybranch structure with a smaller transmission loss. The above is one of such publications. For instance, FIG. 4 of the publication discloses a structure achieved by using a tapered waveguide (with the smaller core width set at W11 and the larger core width set at W12 (=2xc3x97W11)) to guide a light signal entering through an input waveguide to an output waveguide, connecting the input waveguide having a core width of W11 to the side of the tapered waveguide with the smaller core width and connecting two branched waveguides each having a core width W13 (=W11) to the side with the larger core width.
However, a technical problem arises when actually manufacturing the Y-branch structure shown in FIG. 4 of the publication in that the shape of the front end of the wedge that must be formed at the branching portion becomes dull and as a result, has a specific width, to cause an increase in the transmission loss at the branching portion. In order to solve the problem in the Y-branch structure shown in FIG. 4 and reduce the transmission loss at the branching portion, the publication proposes a structure achieved by fitting a wedge structure member at the branching portion thereof, as illustrated in FIG. 1.
Even when the Y-branch structure in FIG. 1 of the publication is adopted, i.e., even when a wedge structure member is fitted at the branching portion, branched waveguides having a width much smaller than the core width are required for the connection to the output waveguide. Thus, even a very slight dimensional error occurring during the production results in a great degree of inconsistency in the characteristics. In addition, the presence of the branched waveguides having a width much smaller than the core width tends to cause a slight defect within the core to manifest as a great degree of inconsistency in the characteristics.
Addressing the problems discussed above, according to the present invention, in an optical integrated circuit device having a Y-branch structure for branching a light signal input through an input waveguide to transmit the branched light signal into two output waveguides, the Y-branch structure satisfies the following requirement.
Namely, the Y-branch structure is provided with a tapered waveguide having an input surface that has a waveguide width xe2x80x9caxe2x80x9d equal to the width of the input waveguide to be connected flush to the input waveguide and an output surface with a waveguide width W ( greater than 2a+g) which is larger than a width 2a+g achieved by adding a gap width g, which is the dimension of the gap between two output waveguides to a width 2a equivalent to the total width of the two output waveguides each having a waveguide width a equal to the width of the input waveguide to be connected with the two output waveguides via a stage.
It is desirable to the tapered waveguide according to the present invention satisfy theta2 less than theta1 less than 2xc3x97theta2 when tan theta2=g/2L and a+2L tan theta1 greater than 2a+g with L representing the length of the tapered waveguide, thetal representing the opening angle created by an inclining surface of the tapered waveguide relative to the central axis of the tapered waveguide and theta2 representing the mounting angle at which an output waveguide is mounted at the tapered waveguide relative to the central axis of the tapered waveguide.
Alternatively, the tapered waveguide according to the present invention should satisfy theta2 less than 1.0xc2x0 when (a+g)/2L less than tan theta1 less than (a+g)/L, with L representing the length of the tapered waveguide, theta1 representing the opening angle created by an inclining surface of the tapered waveguide relative to the central axis of the tapered waveguide and theta2 representing the mounting angle at which an output waveguide is mounted at the tapered waveguide relative to the central axis of the tapered waveguide.